Plural coat enameling process



United States PatentO PLURAL COAT ENAMELING PROCESS Burnham W. King andJohn R. Schultz, Jr., Columbus, Ohio, assignors, by mesne assignments,to The Fletcher Enamel Company, Dunbar, W. Va., a corporation of WestVirginia No Drawing. Application November 15, 1955 Serial No. 547,046

5 Claims. (Cl. 117-70) This invention relates to a process for providinga vitreous enamel coating on a metallic surface and, more particularly,to an improved process for providing a very thin mat ground coat forporcelain enameled steel articles.

Patent applications, Serial Nos. 409,539 and 453,424, now U. S. PatentNo. 2,786,782, describe a method and preferred compositions forproviding thin mat ground coats for porcelain enameling. These groundcoats differ from the conventional ground coats in that they areextremely thin (approximately .0005 to .004 inch thick) and are notglassy in appearance. The advantageous features of this process over themethods now commonly employed are: much smaller amounts of enamelingmaterials are required for a given area; expensive adherence-promotingoxides, such as the oxides of cobalt, nickel, and manganese, areunnecessary ingredients to the frit compositions; and no specificsurface preparation, such as etching or metal plating, other thanproviding a clean surface, is required.

In applying vitreous enamel ground coats by means of prior art methods,including the methods disclosed above, it is considered necessary toprepare an enamel frit, then ball-mill the frit with clay and water plusminor mill additions to obtain the enamel slip with which to coat thearticle. When applying a glassy ground coat, it is necessary to firstfrit the enameling material in order to obtain an even, homogeneousadherent glassy coat. However, it has now been found that the step offritting the enamel ground-coat composition is unnecsssary when applyinga thin, mat-type ground coat of the nature described in patentapplications, Serial Nos. 409,539 and -It is, therefore, the object ofthis invention to provide an improved method of producing porcelainenameled articles.

It is also an object of this invention to provide a method of producingan enameling ground coat whereby the step of fritting enamelingmaterials is eliminated.

Other objects and advantageous features can best be determined by thefollowing specification and examples.

In general, the present invention relates to a method of producing athin mat ground coat which comprises milling a slip directly fromraw-batch enameling materials, omitting the step of first fritting theseenameling materials, applying the slip to a metallic article to a dryweight of 8 to 17 grams per square foot, two sides, and firing to obtaina coating, .0005 to .004 inch thick.

When enameling steel parts by the conventional enameling processes,raw-batch enameling materials, such as fiint, feldspar and borax arebrought together and fritted by heating the materials to a temperatureof about 2200 F. and quenching in water. The frit is then ball milledwith clay, water and other mill additions to form a slip which is thenapplied to a steel article to such a weight or thickness that when fireda glassy ground coat is' obtained. For example, when applyingconventional ground coat to ZO-gage steel sheets, a conventionalenameling slip is applied at a dry weight of from 1% to 1% ounces persquare foot and forms a glass when fired at a temperature of about 1600F. for from about 3 to 5 minutes. In patent application, Serial No.409,539 a process is disclosed whereby the slip is applied to a weightof from /8 to ounce per square foot, dry weight, and fired at from 1300to 1700 F. for from 2 to 8 minutes to form a nonglassy ground coatapproximately .0005 to .002 inch thick. This ground coat may then besatisfactorily provided with a conventional enameling cover coat that isordinarily applied to a glassy-type ground coat. Patent applicationSerial No. 453,424 discloses preferred theoretical enameling fritcompositions when using the process of patent application Serial No.409,539. These preferred ranges are disclosed in Table 1 below. Inobtaining the thin mat ground coat it has been found that the enamelingmaterials employed must be those that will form a glass when fired inthe conventional manner but need not contain the conventional adherencepromoting oxides. This is believed to be due to the fact that inproviding the thin mat coat a conventional glass must first be obtainedbut that due to the thickness of application and the firing temperaturesand times involved, the glass becomes saturated with iron oxide andconverts into the desired mat coating.

'It has now been found that the step of fritting may be eliminated whenemploying the process of patent application Serial No. 409,539.Raw-batch materials may be added directly to the ball mill or grindingapparatus in preparing an enamel slip.

In the practice of the present invention, where the step of fritting isomitted, highly refractoryenameling compositions mustv be avoided. Thenecessary reactions between the raw materials are accomplished duringthe firing of the dried slip which is preferably performed within thetemperature range of from 1200 F. and 1700 F. It is, therefore,essential that the raw-batch materials employed be predominantly made upof materials that will form a glass within the temperature range of from1200 F. to 1700 F. For this reason, a conventional flint-, feldspar-,borax-type raw-batch composition may not be employed in the process ofthe present invention. Flint and feldspar are highly refractorymaterials and may be used as enameling materials for the present processonly in minor quantities.

It is desirable when employing the process of the present invention toavoid using excessive amounts of enameling materials that tend to emitgas during firing. Examples of such materials are the carbonates whichemit carbon dioxide when fired. The emission of excessive amounts of gasadversely affects the smoothness and continuity of the ground coat.

The enameling materials that may be used with the process of the presentinvention are those materials which will melt and form essentially thesame calculated glass compositions as those obtained in conventionalground coats, when fired at a temperature of from about 1200 F. to 17 00F. Adherence-promoting oxides such as the oxides of nickel, manganeseand cobalt are not necessary enameling ingredients but will notadversely affect the resulting ground coat if present in their usualconcentrations.

In the place of the conventional enameling materials such as flint andfeldspar it is advantageous to employ alkali silicates such as sodiumsilicate. Other enameling materials which have proved to be highlysatisfactory are the following:

Kyanite Sodium aluminate Small amounts of refractory enameling materialsmay be employed so long as these materials are not present in suchquantities that a satisfactory glass and mat coating cannot be attainedwithin the firing rangeof from 1200 F. to 1700 F.

These enameling materials are mixed with the usua slip formingmaterials, such as clay and water. Slip materials commonly used to formvitreous enamel may be employed in the present process. When employingclay in the slip the clay should not exceed 15 weight percent of theslip because of the refractory nature of clay. j

The preferred composition ranges disclosed in patent application SerialNo. 453,424 applyequally to the process of the present invention.Therefore, it is preferable that any enameling materials used, includingthe abovementioned materials, on calculated analysis, will fall withinthe preferred ranges. In the present invention these ranges arecalculated enameling. compositions instead of calculated fritcompositions.- Table 1 below describes these preferred ranges.

5101.. 30-45 B103 17-23 N820 and/0r K 0- 11-22 08 ss AlgOs 0 7 Minoradditions of other enameling materials may be added to the raw-batchcompositions and the theoretical calculated compositions of Table 1which do not adversely affect the resulting mat ground coat and which donot throw the composition substantially outside of the preferredconcentration ranges.

Due to the fact that the preferred firing period is of short duration,the reactions must take place rapidly. To aid in this it is desirable tohave intimate contact between fine ground particles. It is advantageousto employ water soluble and finely ground materials. The use of coarsematerials requires longer firing times to complete the necessaryreactions while firing. It is, therefore, advantageous that even theinsoluble enameling constituents possess an average particle size ofabout 200 mesh or less.

The enameling composition is ball milled or mixed with mill additions,such as clay and water, to form a liquid slip for enameling purposes.The liquid slip is applied to a steel surface by spraying, dipping,brushing, or other suitable means to form a coating. This coating isdried before firing. For best results the coating should be applied to aweight of from 8 to 17 grams per square foot dry weight two sides (4 to8 grams per square foot dry weight one side) to form a coating that willbe approximately .0005 to .004 inch thick after firing.

The firing temperature may range from 1200 F. to 1700 F. and the time offiring may vary from 2 to 10 minutes depending on the enamelingmaterials, the temperature used, and the thickness of the metal beingenameled. If the coated steel surface is overfired, the coating willbecome mechanically weak, there may be excessive reaction with a covercoat, there will be a loss of the adhesive-promoting properties of thecoating to the metal surface, and the resulting coat may exhibit a roughsurface. If underfired, there will be a loss in the adhesion propertiesof the coating to the metal surface.

The optimum time and temperature within the ranges given are dependentupon the shape and gage of the 4 steel-basis material, the compositionof the coating and the thickness of the slip is applied.

After cooling to a handling temperature, the enameled articles may begiven a cover coat of a porcelain enamel by coating and firing again inthe conventional manner.

The following specific examples are given to illustrate more fully theprocess of the present invention. They are given merely as illustrationsand are not meant to limit the scope of the invention.

Enameling slips were prepared by ball milling the compositionsillustrated in Table 2, below. The slips thus prepared were applied tosteel surfaces by dipping to form a coating. These coatings, after theyhad dried, weighed approximately 12 grams per square foot of surfacearea, both sides.

All samples were fired for approximately 4 to 6 minutes at a temperatureof approximately 1500 F. All samples were coated with a conventionalenamel cover coat and refired. All of these samples exhibitedcommercially acceptable vitreous enamel surfaces.

Table 2 Batch Composition Material Amount, parts by weight Sodiumsilicate, solution (36.6% solid sodium silicate; 1 NaZO to Sodiumaluminate. Kyanite 1 Additional water added after mixing to adjust todipping consistency. For convenience, the invention has been describedin connection with its use on steel surfaces, but it can be used also onenameling iron, cast iron, and similar ferrous materials.

While certain preferred embodiments of this invention have beendescribed, it will be obvious to those skilled in the art that variousmodifications may be made without departing from the true spirit andscope of the invention.

What is claimed is:

l. The process of producing a vitreous enameled metal body whichcomprises: mixing with clay and water unfritted enameling materials toform a slip, said enameling materials being capable of forming a glasswithin a temperature range of from 1200 F. to 1700 F., and saidenameling materials being in such proportions as to yield, on calculatedanalyses, an enameling material consisting esssentially of:

Weight percent Fe O 1-15 ZnO 0-9 SiO 30-45 B 0 17-28 At least onematerial selected from the group consisting of Na O and K 0 11-22 CaO3-8 A1 0 0-7 applying the resulting slip to a steel body to a dry weightof from 8 to 17 grams per square foot two sides, and firing within atemperature range of from 1200 F. to 1700 F.; applying a vitreous enamelcover coat and refiring.

2. The process of producing a vitreous enameled metal body whichcomprises: mixing unfritted enameling materials selected from the groupconsisting of alkali silicates boraX, boric acid, sodium calcium borate,colemanite, sodium aluminate, kyanite, alumina, aluminum silicate andblack iron oxide, in such proportion as to yield, on calculatedanalyses, a material consisting essentially of:

Weight percent applying the resulting material in an enameling slip to asteel body to a dry weight from 7 to 17 grams per square foot two sides,and firing within a temperature range of from 1200 F. to 1700 F. for aperiod of from 2 to minutes; applying a vitreous enamel cover coat andrefiring.

3. The process of producing a vitreous enameled steel body whichcomprises: preparing a slip by mixing enameling materials with water andup to percent dry weight clay, said enameling materials selected fromthe group consisting of alkali silicates, borax, boric acid, sodiumcalcium borate, colemanite, sodium aluminate, kyanite, alumina, aluminumsilicate and black iron oxide, and said enameling material being in suchproportion and to yield, on calculated analyses, a material consistingessentially of:

Weight percent F5304 ZnO 0-9 SiO 30-45 B 0 17-28 At least one materialselected from the group consisting of Na O and K 0 11-22 CaO 3-8 A1 00-7 applying the resulting slip to a steel body to a dry weight 6 andsaid enameling materials being in such proportion as to yield, oncalculated analyses, when fired at a temperature of from 1200 F. to 1700F. for a period of from 2 to 10 minutes, a glass consisting essentiallyof:

Weight percent Fe O 1-15 ZnO 0-9 SiO 30-45 B 0 17-28 At least onematerial selected from the group consisting of Na i) and K 0 11-22 CaO3-8 A1 0 0-7 silicate, l Na O to 2.94 SiO weight ratio) 27.0 Sodiumsilicate powder (1 Na O to 3.11 S103 weight ratio) 35.0 Regular borax29.8 Sodium calcium borate 31.0 Clay 10.0 Flint 2.0 Feldspar 2.0 BlacklI'OIl oxide 9.1 Kyanite 5.3

Total 151.2

applying the resulting slip to a steel body for a dry weight ofapproximately 12 grams per square foot of surface area, both sides,firing at a temperature of approximately 1500 F. for a period of from 4to 6 minutes; applying a vitreous enamel cover coat, and refiring.

References Cited in the file of this patent UNITED STATES PATENTSRosenburg Jan. 21, 1941 OTHER REFERENCES Andrews: Enamels, Twin CityPrinting Co., 1935, p. 295 relied on.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 2,864,721

December 16, 1958 Burnham W0 King et al,

ed that err ve numbered patent requiri Patent should or appears in theprinted specification read as corrected belo ng correction and that thesaid Letters we Column 5, line 16, for

dry Weight from '7 to 17 grams Weight from 8 to 1'7 grams read me drySigned and sealed this 14th day of April 1959,

(SEAL) Attest:

KARL H, AXLINE ROBERT C. WATSON Attesting Officer Commissioner ofPatents

1. THE PROCESS OF PRODUCING A VITREOUS ENAMELED METAL BODY WHICHCOMPRISES: MIXING WITH CLAY AND WATER UNFRITTED ENAMELING MATERIALS TOFORM A SLIP, SAID ENAMELING MATERIALS BEING CAPABLE OF FORMING A GLASSWITHIN A TEMPERATURE RANGE OF FROM 1200*F. TO 1700*F., AND SAIDENAMELING MATERIALS BEING IN SUCH PROPORTIONS AS TO YIELD, ON CALCULATEDANALYSES, AN ENAMELING MATERIAL CONSISTING ESSENTIALLY OF: